The present invention relates generally to the field of melt-blown media, more specifically media that has reduced density while maintaining structural strength. Such media can provide beneficial application in many uses where desirable material properties include low density and high void volume while maintaining a relatively rigid structure, especially under pressure. Uses for such media include filtration media for various applications such as particle filtration, coalescing of oils and leukocyte filtration. Other uses envisioned include insulation, impact absorbing protective and conformable material, and wicking media for evaporators.
Numerous apparatuses and processes for forming melt blown media comprised of a plurality of substantially continuous filaments currently exist in the prior art. In this art, fiber forming devices or fiberizers such as those described in U.S. Pat. No. 3,825,379 issued to Lohkamp et al. and U.S. Pat. No. 3,825,380 issued to Harding et al. are used to spray filaments of synthetic resinous material toward a collection device. During this process, jets of air or other gases act on the filaments to attenuate such filaments to a comparatively fine diameter and convey the same to the collection device. Fibers continue to build up on the collection device until a mass of fibers of the desired size and morphology is achieved.
Several specific processes have evolved from this general concept. One of these processes is described in U.S. Pat. No. 3,849,241 issued to Buntin et al. It discloses a process die or fiberizer consisting of a die head containing separate passages for the filament material and the attenuating air. During operation, molten resinous material is forced through one or more small holes or nozzles in the die head toward a collection device and is attenuated by air streams positioned on the sides of the material outlet holes. The collection method utilized with this process includes a rotating drum to form a continuous mat. Another of these processes is described in U.S. Pat. No. 4,021,281, issued to Pall. It describes the continuous formation of a melt blown media web onto a rotating drum, being deposited in the form of a tubular web that can be slit into flat media. Another process is exemplified by U.S. Pat. No. 4,240,864 issued to Lin et al. This patent discloses a process die or nozzle block which delivers a plurality of filaments toward a rotating collection device. Associated with the filaments are attenuating air streams which function to attenuate the filaments as they travel toward the collection device. Lin et al. also disclose a press roll for varying the pressure applied to the accumulating fibers on the rotating mandrel so as to provide a filter of varying fiber density. Like the processes of Buntin et al. and Pall, the diameter of the individual filaments in the Lin et al. process is constant throughout the entirety of the media. However, contrary to Buntin et al. and Pall, in the Lin et al. process, the resultant media are continuously urged off the rotating mandrel via the noncylindrical press roll to produce a coreless depth filter element.
Another specific process is represented by U.S. Pat. Nos. 4,594,202 and 4,726,901, both issued to Pall et al. Similar to the processes described above, the Pall process includes a fiberizer or fiberizer die having a plurality of individual nozzles through which the molten filament resin is forced toward a collection mandrel. Also similar to the other processes described above, this process discloses the use of air or gas streams for the purpose of attenuating the filaments as they travel toward the collection mandrel. This process differs from the processes described above, however, in that it discloses a means for varying the fiber diameter throughout the radial dimension of the filter element, while maintaining a substantially constant voids volume for each level of fiber diameter variance. Pall et al. accomplish this by sequentially altering certain parameters which affect the fiber diameter during collection of the filaments on the rotating mandrel.
Although each of the above specific processes is generally acceptable for certain applications, each also has certain limitations. For example, one limitation of the Pall et al. (U.S. Pat. Nos. 4,594,202 and 4,726,901) process is that it is a non-continuous or semi-continuous process. In other words, a filter element of finite length is formed by building up a mat of attenuated filaments on a rotating mandrel. When the collected filament material reaches a desired thickness, the filter structure is removed and the process is commenced again for the next filter element.
Although the Pall et al. patents (U.S. Pat. Nos. 4,594,202 and 4,726,901) contemplate a depth filter element comprised of filaments with varying diameters, there are several limitations which exist. First, the process of Pall et al. is not a continuous process, but must be repeated for each filter manufactured. Second, although some filter elements of Pall et al. have filaments of varying diameters, the process of making such elements has limitations. Specifically, the filament diameter is varied by sequentially changing one of several operating conditions of the filament producing mechanism. Whenever such a change is introduced, however, the system takes time to respond to such changes before again reaching equilibrium. The time frame for response is proportional to the degree of change. Because these changes are introduced during the manufacture of each individual filter element, a less stable and more variable process results. Further, the changeover from a filament of one diameter to that of another occurs gradually as a time related transition, rather than abruptly such as where the structure is comprised of two or more discrete filaments.
An important attribute of the media structure is the percent void volume which is the ratio of the air volume in the structure to the total media structure volume. The percent voids volume in the melt-blown media should be as high as possible in order to achieve a number of desirable characteristics in filtration applications, such as high dirt holding capacity and lower initial pressure drop. Generally, achieving a high void volume results in lowering the density of the media mass. It is also desirable to lower the density of a media mass, because a lower density media requires less material usage, allowing for lower material costs, higher throughput, and faster production.
Another advantage of media with high void volume is that they are amenable to insertion of a significant percentage volume of active particles or fibers without inducing an unacceptable increase in pressure drop in filtration applications. For example, activated carbon particles may be dispersed in the media as they are formed. Moreover, masses with high void volumes and lower densities also generally provide advantages for other applications such as thermal insulation, evaporative wicking and impact absorption material.
However, in prior art melt blown media, there is an upper limit beyond which further increasing the percent voids volume becomes undesirable. Attempts to produce low density, high void volume, media structures using the prior art teachings result in reduced fiber-to-fiber bonding and typically insufficient structural strength. As the voids volume is increased in prior art structures, the fibrous media used in a depth filter are more readily compressed by the pressure drop generated by the fluid passing through it. This is particularly troublesome when the fluid is viscous. If the percent void volume is too high, the filter medium will begin to collapse at too low a differential pressure. As it collapses, the pores become smaller and the differential pressure increases, causing still more compression. The resulting rapid increase in pressure drop thus reduces the media's useful life and dirt holding capacity rather than—as might otherwise be expected with the increased void volume media—extending it. Use of a very low density (high voids volume) can also make the filter very soft and thereby more readily damaged in normal handling and more likely to compress and collapse in use.
A drawback of the prior art products is that the low density filters often are made using fine fibers and therefore have a fine micron rating, which is inherent to the finer fiber matrix. It would be desirable to use fine fibers to achieve low density, while maintaining the capacity to produce media with a larger pore structure. For a filtration application, this would mean a coarser micron rating, thereby allowing for filtration of a wider range of particles without premature clogging of the filter. This would require that the fine fiber network is somehow fixed in a more open structure, thereby avoiding the natural packing tendency of the fine fibers that inherently create a finer pore structure.
Although prior art methods exist for manufacturing melt blown media, each of the methods, as well as the products constructed from such methods, have limitations of compressive strength at low media densities. Accordingly, there is a need in the art for an improved, cost efficient melt blown media. A need also exists for a continuous method and apparatus for producing such media.